The effects of skin aging on the appearance of the human face are well studied and documented in dermatology. Each individual's skin aging progression is dependent on both intrinsic and extrinsic factors. Intrinsic factors, such as gender, race, and skin pigmentation, are genetically programmed and unique for each individual and can affect the rate of dermal thinning, loss of mechanical elasticity, and other well-characterized histological and bio-mechanical changes with age. Intrinsic factors affect both sun-protected and sun-exposed body sites. Extrinsic factors include an individual's diet, lifestyle, skin care habits and sun exposure history. Chronic sun exposure is well-known to accelerate the onset time and severity of skin aging. All exposed body sites including the face have some degree of skin photoaging. (Gilchrest., B. Photodamage, Blackwell Science, Inc. 1995).
One of the most visually prominent features of photoaged skin is a mottled and irregular pigmentation that appears as a spot with dark brown coloration on the skin (Griffiths C. E. M., “The clinical identification and quantification of photodamage,” Brit. J. Derm., Vol. 127 (Suppl. 41), 37-42, 1992; K. Miyamoto et al., “Utilization of a high-resolution digital imaging system for the objective and quantitative assessment of hyperpigmented spots on the face,” Skin Research and Technology, Vol. 8, No. 2 pp: 73-78, May 2002, hereinafter the “Miyamoto reference”). These hyperpigmented lesions are called age spots, liver spots, lentigo senilis, or actinic lentigines. Hyperpigmentation in photodamaged skin can be better visualized using methods that reveal subsurface pigmentation not visible with standard white light. One method, called UV-excited fluorescence photography, which was originally introduced by Kollias (Kollias et al., “Fluorescence photography in the evaluation of hyperpigmentation in photodamaged skin”, J Am Acad Dermatol., Vol. 36, pp: 226-230, 1997), involves imaging the skin under narrow-band UVA centered at 365 nm. Epidermal melanin absorbs strongly in this UVA range, approximately 3-5 times its absorption in the visible spectrum. Any UVA that is not absorbed by epidermal melanin enters the dermis where it is scattered and absorbed by collagen and elastin fibers which convert some of the absorbed energy to fluorescence. The wavelength of maximum collagen emission occurs in the visible spectrum, centered at 420 nm. The in vivo absorption of melanin at 420 nm is two times greater than at 540 nm. Thus, the total amount of UVA that enters the skin and reaches the dermis is attenuated by epidermal melanin approximately 5-fold and the amount of visible fluorescence is attenuated by the same epidermal melanin approximately 2-fold. In other words, epidermal melanin detection with UV-excited fluorescence is about 10 times more sensitive compared to visible light. This enhancement in sensitivity allows for the detection of hyperpigmented spots that cannot be seen under normal white light imaging methods. Hyperpigmented spots that cannot be observed with visible light will, without intervention, become darker and more visibly apparent under normal visible light at a later point in life.
Other prominent features of aged skin are rough texture and skin wrinkles (Leyden J. J. “Clinical features of ageing skin”, Br. J. Dermatol. Vol. 122, Suppl. 35, pp: 1-3, 1990) caused in part by the gradual alteration and loss of dermal connective tissues such as collagen, especially in sun-exposed areas of the body (Bailey, Molecular mechanisms of aging in connective tissues, Mech. Aging Dev., Vol. 122, No. 7, pp.: 735-755, 2001). Hyperpigmentation, wrinkles and rough texture are visible skin features that play an important role in the overall appearance and healthiness of skin.
It is of practical value to be able to accurately simulate the aging process. Aging simulation has several useful applications such as computer animation, facial recognition, missing person identification, entertainment, medicine and cosmetics. Various models have been employed to enable the realistic simulation of an aging face including geometric-models, physically-based models, image-based models or bio-mechanical models (Hussein, K. H, Toward realistic facial modeling and re-rendering of human skin aging animation, Proceedings of the Shape Modeling International 2002, IEEE Computer Society, 2002). Attempts have been made to customize aging simulation so that it more accurately depicts a particular person's future aged appearance. For example, aging algorithms have been developed based on a population cohort of images combined with published data regarding facial changes associated with aging in order to simulate an aged appearance of an individual (Hysert P E et al. “At Face Value”: age progression software provides personalized demonstration of the effects of smoking on appearance,” Tobacco Control, Vol. 12, pp: 238-240, 2003). A limitation of this method is that the aged image is a reflection of population norms, and does not necessarily reflect the individual's unique aging process.
Boissiux et al. developed an image-based model for simulating skin aging whereby generic masks of pre-computed wrinkles are applied as textures on a 3D model of a person's face. Eight basic masks are employed and the particular mask used is matched to the person's gender, shape of face and type of expression being simulated (Boissiux et al. “Simulation of skin aging and wrinkle with cosmetic insight”, Computer Animation and Simulation, pp 15-27, 2000). This approach, because it relies on population means, is limited in its ability to accurately predict each person's unique skin features that will appear with age.
Zhang et al. describes a method for transferring the geometric details of an old face onto that of a young face in order to make the young face look old (Zhang et al. “System and method for image-based surface detail transfer” US7020347B2, 2006). Conversely, the surface details of a young face can be transferred to that of an old to make an old face look young. This approach is limited by the fact that the aging features of the old face will not be exactly the same features that the young face will eventually realize.